1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image on a recording medium. Examples of the image heating apparatus include a fixing apparatus for fixing an unfixed image on a recording medium and a gloss application apparatus for increasing gloss level of a fixed image on a recording medium by heating the image.
2. Description of the Related Art
In electrophotographic image forming devices, various systems for fixing unfixed toner images are known.
In the various systems, a belt fixing apparatus capable of increasing the size of a fixing nip portion in response to demands for high-speed image formation is proposed (see, for example, Japanese Patent Laid-Open Nos. 8-166734 and 10-319772).
The fixing apparatus has a structure in which a pressure belt is pressed into contact with a fixing roller, and a pressure pad is pressed against the fixing roller from the inner surface of the pressure belt. This structure can provide a long fixing nip portion extending from the pressure pad to a belt suspension roller.
In the fixing apparatus, the fixing roller is rotatably driven by a driving source, and the pressure belt is rotated by following movement of the fixing roller due to sliding friction force produced by sliding on the fixing roller. When a recording medium is present on the fixing nip portion, the pressure belt mainly receives transfer force via the recording medium. Therefore, the peripheral speed of the pressure belt depends on the conveying speed of the recording medium.
However, for a structure in which the pressure belt is rotated by following movement of the fixing roller, the conveying force provided to the pressure belt varies according to the type of the recording medium, environmental conditions, the type of a toner image. This may result in unstable rotation of the pressure belt.
For example, in the case where a large amount of unfixed toner remains over substantially the entire surface of the recording medium, when the recording medium enters the fixing nip portion, the coefficient of kinetic friction between the fixing roller and the recording medium tends to decrease, and the conveying force of the pressure belt decreases. As a result, the recording medium lags behind the fixing roller and the pressure belt slips on the fixing roller, and poor imaging (e.g., displacements of an image) occurs. In this case, the peripheral speed of the pressure belt is substantially the same as that of the conveying speed for the recording medium.
As described above, existing pressure-belt driving systems cannot offer high image quality.
With aim of preventing a recording medium from lagging, an apparatus including an override mechanism used as a driving mechanism is disclosed in Japanese Patent Laid-Open No. 2-222980. However, this mechanism is insufficient for completely solving the problems.
The system using this override mechanism has a structure in which, when a recording medium is not present on the fixing nip portion, the pressure belt is rotated by following movement of the fixing roller due to sliding friction force to the fixing roller, as in the case of the systems disclosed in Japanese Patent Laid-Open Nos. 8-166734 and 10-319772 mentioned above. In this structure, the pressure belt receives driving force only when the recording medium (toner image) slips on the fixing roller and the pressure belt lags behind the fixing roller. In other words, it takes time, however small, to change from when the peripheral speed of the pressure belt becomes lower than that of the fixing roller to when the pressure belt receives the driving force.
As a result, the speed of the pressure belt is changed in the course of fixing the toner image on the recording medium, and the speed change causes poor imaging, such as image displacements.